Electric utility companies and power consuming industries have in the past employed a variety of approaches to metering electrical energy. Typically, a metering system monitors power lines through isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs are then selectively treated to determine the particular type of electrical energy being metered. Because electrical uses can vary significantly, electric utility companies have requirements for meters configured to analyze several different nominal primary voltages. The most common of these voltages are 120, 208, 240, 277 and 480 volts RMS. Presently, available meters have a different style for each of these applications, both electro-mechanical and electronic. This forces the electric utility companies to inventory, test and maintain many different styles of meters. Consequently, a need exists for reducing the number of meter types a utility need inventory by providing a meter capable of operation over a wide dynamic range.
The problem of wide amperage dynamic range was addressed in U.S. Pat. No. 3,976,941--Milkovic. It was there recognized that solid state electronic meters were becoming more desirable in metering applications, however, such solid state meters had a critical drawback in their amperage dynamic range. An effort was described to improve the amperage dynamic range of solid state meters so that such meters would be operationally equivalent to prior electro-mechanical meters. The problem with such meters, however, was their failure to address the multiple voltage situation. Utility companies utilizing such meters would still be forced to inventory, test and maintain many different styles of meters in order to service the various voltages provided to customers.
It has been recognized in various meter proposals that the use of a microprocessor would make metering operations more accurate. It will be understood, however, that the use of a microprocessor requires the provision of one or more supply voltages. Power supplies capable of generating a direct current voltage from the line voltage have been used for this purpose. Since electric utility companies have requirements for various nominal primary voltages, it has been necessary to provide power supplies having individualized components in order to generate the microprocessor supply voltages from the nominal primary voltage.
Consequently, a need exists for a single meter which was capable of metering electrical energy associated with nominal primary voltages in the range from 96 to 480 volts RMS. Applicants resolve the above problems through the use of a switching power supply and voltage dividers. It will be recognized that switching power supplies are known. However, the use of such a power supply in an electrical energy meter is new. Moreover, the manner of the present invention, the particular power supply construction and its use in an electrical energy meter is novel.
It will also be noted, in order to solve the inventory problem, designing a wide voltage range meter in the past involved the use of voltage transformers to sense line voltage. A significant problem associated with the use of such transformers was the change in phase shift and the introduction of non-linearities that would occur over a wide voltage range. It was not easy to remove such a widely changing phase shift or to compensate for the non-linearities.
Consequently, a need still exists for a single meter which is capable of metering electrical energy associated with nominal primary voltages that also minimizes phase shift in the voltage sensors over a wide voltage range.